1. Field of the Invention
This invention relates to a flat filament for an X-ray tube, and an X-ray tube.
2. Description of the Prior Art
An X-ray tube used in an X-ray apparatus for medical purposes includes a cathode for generating an electron beam, and an anode for generating X-rays upon collision with the electron beam generated from the cathode. The cathode uses a flat filament having an electron emission surface and a pair of current supply legs connected to the electron emission surface (Japanese Unexamined Patent Publication H5-67442 and the specification of U.S. Pat. No. 6,115,453).
FIG. 5 is a schematic view of such a conventional flat filament 71.
This flat filament 71 includes an electron emission surface 72 having two bent parts, and a pair of current supply legs 73 and 74 connected to the electron emission surface 72. The flat filament 71, with the pair of current supply legs 73 and 74 bent at right angles in positions shown in dashed lines in FIG. 5, is attached to a focusing electrode.
FIG. 6 is a schematic view of a flat filament 75 described in the specification of U.S. Pat. No. 6,115,453.
This flat filament 75 is provided to secure a longer lifetime, and includes an electron emission surface 76 having a plurality of regions in the shape of concentric circles, and a pair of current supply legs 77 and 78 connected to the electron emission surface 76. This flat filament 75 also, with the pair of current supply legs 77 and 78 bent at right angles in positions shown in dashed lines in FIG. 6, is attached to a focusing electrode.
An X-ray tube which uses each of the flat filaments noted above can obtain only a focus of fixed size corresponding to the size of the electron emission surface of the flat filament. Therefore, when such flat filament is used, it is difficult to realize an X-ray tube having a plurality of focus sizes.
With an X-ray apparatus for medical purposes, on the other hand, it is preferable to change focus sizes according to patients to be X-rayed. That is, it is preferable to reduce the focus size for X-raying minute sites, for example. When X-raying a patient of large body thickness, or when conducting X-raying with a reduced load on the anode, it is preferable to enlarge the focus size.
Therefore, use is made also of an X-ray tube having a plurality of filaments arranged in the single X-ray tube, but this poses a problem of complicating the construction. With an X-ray tube of the envelope rotation type having an envelope containing a cathode and an anode, the anode being rotatable with the envelope, it is necessary to place a filament at the center of rotation of the anode and envelope. This limits installation of the filament to only one location, and hence a problem that a plurality of filaments cannot be arranged.